Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called “membrane-electrode-assembly” comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton: conductive material intermingled with the catalytic and carbon particles. One such membrane-electrode assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode. Flow fields are provided for distributing the fuel cell's gaseous reactants over surfaces of the respective anode and cathode. The electrical contact elements may themselves form a part of the flow field in the form of appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 & O2) over the surfaces of the respective anode and cathode.
A fuel cell stack comprises a plurality of the membrane-electrode-assemblies stacked together in electrical series. The membrane-electrode-assemblies are separated from one, another by an impermeable, electrically conductive contact element, known as a bipolar plate. The bipolar plate has two major surfaces, one facing the anode of one cell and the other surface facing the cathode on the next adjacent cell in the stack. The plate electrically conducts current between the adjacent cells. Contact elements at the ends of the stack contact only the end cells and are referred to as end plates.
In a PEM fuel cell environment that employs H2 and O2 (optionally air), the bipolar plates and other contact elements (e.g., end plates) are in constant contact with acidic solutions (pH 3 to 5).
In addition, the fuel cell operates at elevated temperature on the order of 60° C. to 100° C. Moreover, the cathode operates in a highly oxidizing environment, being polarized to about +1 V (in comparison to a normal hydrogen electrode, i.e., the anode) while being exposed to pressurized air. The anode is constantly exposed to a harsh environment of pressurized hydrogen. Hence, many of the conventional contact elements are made from metal and must be resistant to acids, oxidation, and hydrogen embrittlement in the fuel cell environment. Metals which meet this criteria are costly. One proposed solution has been to fabricate the contact elements from graphite, which is corrosion-resistant, and electrically conductive, however, graphite is quite fragile and difficult to machine.
Lightweight metals such as aluminum and titanium and their alloys, as well as stainless steel, have also been proposed for use in making fuel cell contact elements. Such metals are more conductive than graphite, and can be formed into very thin plates. Unfortunately, such lightweight metals are susceptible to corrosion in the hostile fuel cell environment, and contact elements made therefrom either dissolve (e.g., in the case of aluminum), or form highly electronically resistive, passivating oxide films on their surface (e.g., in the case of titanium or stainless steel) that increases the internal resistance of the fuel cell and reduces its performance. To address this problem it has been proposed to coat the lightweight metal contact elements with a layer of metal or metal compound which is both electrically conductive and corrosion resistant to thereby protect the underlying metal. See for example, U.S. Pat. No. 5,624,769 by Li et al., which is assigned to the assignee of the present invention, and discloses a light metal core, a stainless steel passivating layer atop the core, and a layer of titanium nitride (TiN) atop the stainless steel layer.
Another type of contact element, a bipolar plate, is molded from a polymer resin and has a conductive carbon or graphite powder embedded therein for electrical conductivity. Such material is typically 80% carbon and 20% polymer on a weight basis. Since these materials cannot be fabricated as thin metal substrates, the volumetric power density of stacks using these plates is usually low and they are not widely used. Examples of such composite plates can be found in U.S. Pat. Nos. 6,096,450, 6,103,413 and 6,248,467. Still another type of plate is graphoil, exfoliated graphite, flake material processed as a graphite plate embossed to a final shape and impregnated with a resin. Such material is typically 99% carbon and 1% resin filler.
Accordingly, so-called bipolar plates are used in all types of fuel cells and form both a closure impermeable to gas and liquids for a respective cell and also, with a stacked arrangement of cells, an electrical connection between adjacent cells, so that the positive side of the one cell is simultaneously the negative side of the adjacent cell, which is the reason for the name “bipolar plate”.
As mentioned earlier, problematic in such bipolar plates is the fact that they are subject to corrosion in the environment of the fuel cell, with corrosion producing substances being present in all types of fuel cells.
At the present time, such bipolar plates are provided with a corrosion-resistant layer of a noble metal, such as gold or platinum. Such layers of noble metals are admittedly corrosion-resistant and simultaneously provide the required conductivity. However, they are expensive.
Bipolar plates are also known which are manufactured from graphite and graphite/plastic mixtures, such as are described in EP-A-0933825. These are, however, often brittle materials. If desired to machine these materials and to use them in a fuel cell, the plates must have a certain thickness, which is disadvantageous with respect to the power-to-weight ratio of a fuel cell stack and thus also has an effect on the use of fuel cells in mobile applications, for example, as a source of motive power for a vehicle. The use of plates of graphite and graphite/plastic mixtures is accordingly likewise associated with disadvantages in certain aspects.